1. Field of the Invention
This invention is related, generally, to drilled shafts. More particularly, it relates to a method that enhances the foundations of structures.
2. Description of the Prior Art
Drilled shafts are large-diameter cast-in-place concrete structures that develop enormous axial capacity to resist heavy loads from tall buildings, bridges, and the like. They develop such capacity from a combination of side shear resistance (sometimes called skin friction) and end bearing capacity (also called tip capacity). However, the end bearing capacity is rarely fully utilized due to the large displacement required to develop it.
What is needed, then, is a method that reduces the amount of displacement required to develop end bearing capacity, thereby increasing the usable overall capacity.
One prior art method (Beck et al.) provides similar improvements, but differs from the novel method in four points: (1) The Beck et al. method results in an end bearing that is about twice that of an unimproved foundation; (2) It uses pressurized grout that cures under pressure in the base of the foundation; (3) It may unwittingly leave the foundation at the brink of failure; and (4) It discloses the use of grout and no other material to perform end bearing improvement.
Thus there is a need for a method where the end bearing capacity is not limited to twice the capacity of an unimproved foundation, where grout subjected to external additional pressure that is locked-in while it cures is not used in the base of the foundation, that does not place the foundation in danger of failure, and that does not employ only grout to perform end bearing enhancement.
In the prior art, end bearing enhancement from pressure grouting with hardening fluids is limited to free-draining soils that are compressible in the time frame associated with the chemical reaction of hardening grout.
There is a need, therefore, for a method that enables enhancement of end bearing capacity in poorly draining soils by allowing more time to squeeze water from voids in such soil.
Studies have shown that by merely preloading and unloading a foundation, the resulting stiffness when reloaded is dramatically improved (Mullins, et al. 1999, 2000, 2004, 2006). Stiffness is the ratio of applied load to resulting displacement. As such, foundation capacities are influenced by the act of loading and reloading as well as the degree to which it is loaded. Methods that cause preloading are therefore considered desirable. However, in the case of post-grouting deep foundations (injecting high pressure grout beneath the tip), studies (Frederick, 2001, Mullins, 2004) have further shown that similar end bearing enhancement is obtained when grout cures with additional external pressure is locked-in or when cured under at-rest hydrostatic pressure. Locked-in pressure is achieved by closing a permanent in-line valve at the top of the shaft once the maximum or desired design pressure is obtained. These two methods function differently, but the resultant improvement is about the same when the shaft is tipped in free draining soils.
However, when tipped in rock or other fracturing bearing strata, the net result of the two methods is quite different; maintaining additional pressure while curing can lead to catastrophic failure. Thus, the performance enhancement of pressure grouted foundations that utilizes only the increased stiffness associated with preloading and subsequently reloading a foundation element is considered to be safer.
There is a need, therefore, for a method that improves the end bearing capacity of deep foundations without using grout and that cures while subjected to locked-in external/additional pressure.
However, in view of the prior art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the pertinent art how the identified needs could be fulfilled.